This invention relates generally to alternating current machine drive systems, and more specifically, to a transistorized alternating current machine drive system which is regulated in accordance with inverter output current to provide maximum machine torque at minimum peak harmonic current.
In alternating current machine applications where regulation of machine speed and torque is desired, inverter machine drive systems are employed. Typically, an inverter machine drive systems comprises an alternating current machine, either a synchronous or induction type machine, which is supplied with alternating current from a direct current source, usually a phase controlled rectifier, by an inverter. Such inverters are commonly configured of a plurality of pairs of switching devices, with the switching devices of each pair coupled in series-aiding fashion and each of the pairs of serially-coupled switching devices coupled across the direct current source and connected at the junction between serially coupled switching devices to a respective machine phase. When the switching devices of each pair are alternately rendered conductive in sequence, alternating current is supplied by the inverter to the machine. By conditioning the alternating current supplied by the inverter, that is, by varying the frequency and duration of inverter switching device conduction, machine speed and torque, respectively, can be controlled accordingly.
Heretofore, each of the switching devices of each pair of inverter switching devices has usually been comprised of a thyristor. The application of high current transistors to alternating current machine drive system inverters has been limited because of the extreme sensitivity of present day transistors to excessive peak currents. A typical transistorized inverter-alternating current machine drive system subjects inverter transistors to peak currents of a magnitude in excess of twice the dc inverter link or energizing current. Unfortunately, inverter transistors cannot be protected against such peak currents by fuses, thus requiring that inverter link current to be limited to protect inverter transistors, thereby limiting inverter operating range.
The present invention concerns a transistorized inverter-machine drive system which is regulated in accordance with inverter phase current so that peak inverter currents are limited while machine torque is maximized, thereby providing satisfactory machine drive system performance. By regulating the inverter-machine drive system in accordance with inverter phase current, sensitivity of the machine drive system to machine parameter variations is minimized.